The present invention is directed to an improvement in a device for the exact positioning of a movable part. It is particularly applicable to the exact positioning of a read and write head on a given track of a magnetic disc on which data is recorded.
Devices exist which serve to provide the displacement of movable parts according to a given rule of motion to bring them into a determined position of their trajectory. Such a device has been described in U.S. Pat. No. 3,729,668, granted Apr. 24, 1973. It particularly includes energizing media for the motor whose task is the displacement of the movable part. These media are controlled by a signal indicative of the difference between the real speed of the part, and the reference speed defined by the chosen rule of movement. This reference speed is given a constant low value as soon as the remaining distance for the part to travel becomes less than a given value, that is, when the part is close to its destination. By avoiding an abrupt deceleration of the part at the end of its course therefore the device permits a much greater rapid stabilization at the stopping point by serving to exactly position said part. In a copending United States application entitled "Precisely Positioning a Movable Member", application Ser. No. 215,967, filed Dec. 30, 1971, and assigned to the assignee of the present invention, the means are described by which several signals of sinusoidal voltage are generated, whose zero passages represent the passages of a write and read head on the different tracks of a disc. Logical pulses, generated by the zero passages of these signals, are sent to a difference of address register indicating the difference between the track to be reached and the track arrived at by the served head. This register then makes it possible to send a signal to the servo circuit for exact positioning, when the head is on the recording track to be attained. Thus, when the previously selected track is reached, the servo circuit for exact positioning compares a signal representing the real speed of the head with a reference signal originating from a multiplexer. The error signal at the output of the servo circuit permits modification of the energizing of the motor, which displaces the head until its stoppage on the desired track. Though such a device for the exact positioning of a movable part reduces to a substantial extent the number of oscillations of the head in question, about its position of equilibrium, the time required for its stabilization is considerable.